Intraocular pressure can be measured by several different instruments, each based on slightly different physical principles and suited to different clinical situations. While Goldmann applanation tonometry (GAT) remains the gold standard, the COA exam expects you to understand all common tonometry methods — when to use each, what preparation is required, and what factors limit accuracy.
This guide compares the five tonometry methods most relevant to COA exam preparation: GAT, non-contact tonometry (NCT), iCare rebound tonometry, the Tono-Pen, and the Perkins handheld tonometer. Understanding their differences helps you select the right tool for the right patient and interpret readings appropriately.
Tonometry Methods at a Glance
| Method | Principle | Anesthetic? | Fluorescein? | Accuracy | Best For |
|---|---|---|---|---|---|
| GAT (Goldmann) | Applanation — 3.14 mm area | Yes | Yes | Gold standard | Definitive IOP, glaucoma management |
| NCT (Air Puff) | Air pulse + IR reflectance | No | No | Good for screening | High-volume screening, no drop authorization |
| iCare Rebound | Probe rebound speed | No | No | Good; less than GAT | Children, anesthetic allergy, portable |
| Tono-Pen | Handheld applanation | Yes | No | Moderate | Irregular corneas, non-slit-lamp positions |
| Perkins | Handheld applanation (GAT prism) | Yes | Yes | Near GAT-level | Supine patients, no slit lamp available |
Goldmann Applanation Tonometry (GAT)
GAT is mounted on the slit lamp and uses a split biprism to flatten exactly 3.14 mm of the cornea. At that precise applanation area, tear film surface tension and corneal rigidity cancel each other, making the measurement self-correcting for eyes with average corneal properties. The force on the drum (multiplied by 10) equals the IOP in mmHg.
Indications
- Definitive IOP measurement for glaucoma management
- Confirmation of elevated NCT readings
- All routine slit lamp examinations
- Pre- and postoperative IOP assessment
Contraindications
- Active corneal infection or ulceration
- Anesthetic allergy (proparacaine or tetracaine)
- Ruptured globe or penetrating injury
- Soft contact lenses in place (fluorescein stains)
Non-Contact Tonometry (NCT / Air Puff)
NCT emits a calibrated air pulse that transiently applanates the cornea. An infrared system detects the moment of maximum corneal flattening (maximum reflectance) and converts the air pressure at that moment to an IOP estimate. The measurement takes approximately 3 milliseconds — sub-perceptible in duration, though many patients anticipate and blink before the trigger fires.
Key advantage: No anesthetic, no fluorescein, no probe contact
This makes NCT the preferred first-line screening method in settings where speed and throughput are priorities, and in offices where ophthalmic staff do not have authorization to instill medications.
Key limitation: Must confirm elevated readings with GAT
Any NCT reading exceeding 21 mmHg — or any reading with high inter-measurement variability (greater than 3–4 mmHg spread) — requires GAT confirmation. NCT frequently overestimates IOP in anxious or blink-prone patients.
iCare Rebound Tonometry
The iCare tonometer uses a lightweight, magnetized disposable probe that makes brief contact with the central cornea. An electromagnetic field accelerates the probe toward the eye, and the speed at which it bounces back (rebounds) is measured by the instrument. Higher IOP causes the probe to decelerate more rapidly and rebound faster; lower IOP causes slower deceleration and a slower rebound. The instrument averages six measurements automatically and displays the result.
Because the probe tip is so small and the contact duration is sub-millisecond, most patients do not feel the measurement. This makes iCare uniquely useful for pediatric patients, patients with anesthetic allergies, and home monitoring scenarios (the iCare HOME model allows patient self-testing).
Contact Duration
<1 ms
Sub-millisecond; rarely felt
Measurements Averaged
6
Auto-averaged per reading
Anesthetic Required
No
Key advantage for pediatrics
iCare: Important Technique Points
- Hold the probe perpendicular to the cornea — angled contacts reduce accuracy significantly
- Aim for the central 3 mm of the cornea, avoiding the visual axis or corneal periphery
- The patient should look straight ahead at a distance target — convergence raises IOP slightly
- Discard readings flagged with an error indicator (probe hit at wrong angle or distance)
- Use disposable single-use probe tips for infection control
Tono-Pen (Handheld Applanation Tonometry)
The Tono-Pen XL is a handheld electronic applanation tonometer. It uses a small transducer-mounted tip to applanate the central cornea through a disposable latex cover (LatexFree covers are available). The device performs multiple measurements and displays the average IOP along with a confidence interval expressed as a percentage — the lower the percentage, the tighter the agreement among measurements and the higher the confidence in the result.
Best Use Cases
- Eyes with irregular corneal surfaces (keratoconus, grafts, scars)
- Patients unable to sit at a slit lamp (wheelchair, OR table)
- Postoperative filtering blebs (measure around the bleb)
- Children under sedation or in the operating room
- Emergency department IOP checks
Limitations
- Less accurate than GAT on normal corneas (±3 mmHg)
- Requires topical anesthetic (unlike iCare)
- Latex cover must be replaced between patients
- High IOP values tend to be underestimated
- Confidence interval must be acceptable (≤5%) for clinical use
Perkins Handheld Applanation Tonometry
The Perkins tonometer uses the same GAT prism and the same Imbert-Fick applanation principle as the slit-lamp-mounted Goldmann tonometer, but in a portable, handheld format. This makes the Perkins the closest portable equivalent to GAT. The prism is illuminated internally with a cobalt blue LED, and the practitioner aligns the mires while viewing through a magnifying eyepiece on the instrument itself — no slit lamp is needed.
Because the Perkins uses the same physical applanation as GAT, accuracy on normal corneas is comparable. However, it requires the same preparation (topical anesthetic and fluorescein) and the same technique skill as GAT. Its primary advantage is the ability to use it in any patient position.
Supine IOP measurement
The Perkins is the preferred method for measuring IOP in anesthetized patients in the operating room, patients who cannot sit upright, and children under general anesthesia. Note that IOP is slightly higher in the supine position due to increased episcleral venous pressure from head-down positioning.
Same prism care as GAT
The Perkins prism must be disinfected between patients using the same protocol as the GAT prism — alcohol wipe, minimum 5 minutes dry time, or use of disposable prism covers. The prism is interchangeable with standard GAT prisms on some models.
Positional IOP difference
IOP measured in the supine position is typically 1–3 mmHg higher than when measured seated. When interpreting Perkins readings on supine patients and comparing to GAT readings taken in the seated position, account for this systematic positional difference.
Practice Tonometry Instrument Questions
Opterio includes instrument comparison and selection questions within the COA Assessments domain, with AI-powered explanations that teach the clinical reasoning for each answer.
Patient Preparation Summary
| Preparation Step | GAT | NCT | iCare | Tono-Pen | Perkins |
|---|---|---|---|---|---|
| Topical anesthetic | Required | Not needed | Not needed | Required | Required |
| Fluorescein | Required | Not needed | Not needed | Not needed | Required |
| Slit lamp required | Yes | Separate instrument | No | No | No |
| Soft CLs: remove first? | Yes (fluorescein stains) | Optional | Yes (probe contact) | Yes (anesthetic used) | Yes (fluorescein stains) |
| Disinfection needed | Prism must be disinfected | No eye contact | Disposable probe tip | Disposable cover | Prism must be disinfected |
Documentation Requirements
Regardless of the tonometry method used, consistent documentation is essential for tracking IOP over time and ensuring clinical decisions are based on comparable readings.
Always specify the method
Write "IOP OD 18 mmHg OS 16 mmHg by GAT" or "by NCT" or "by iCare" — never just a number. Different tonometers are not interchangeable numerically.
Record the time
Diurnal IOP variation of 3–6 mmHg makes time an essential part of every IOP record. Comparing a morning reading to an afternoon reading without noting the time introduces interpretive error.
Note patient position for Perkins
If the Perkins was used with the patient supine, document this. Supine IOP is typically 1–3 mmHg higher than seated IOP and should not be directly compared to historical seated measurements without accounting for the positional difference.
Flag NCT readings for GAT confirmation
If NCT exceeds 21 mmHg, add a note: "NCT OD 24 mmHg — GAT requested." This communicates clearly to the provider and ensures the reading is followed up before treatment decisions are made.